prepared for cavernosography."/>
Figure 15.21 Bull prepared for cavernosography. Note suture placed through the abdominal skin and under retractor penis muscles to pull the penis away from the abdomen (a). As the bull is positioned, an extension set is attached to a hypodermic needle placed into the CCP of the free portion of the penis, the penis is held in extension with a towel clamp placed under the dorsal apical ligament, and traction is applied to the preplaced sutures under the retractor penis muscles (b).
Formation of vascular shunts is recognized as a potential complication following rupture of the tunica albuginea as described under the section on penile hematoma. Anastomoses which develop following disruption of the integrity of the tunica albuginea and subsequent penile hematoma formation are located at the site of injury, the dorsum of the penis at the level of the distal bend of the sigmoid flexure (Figure 15.22). Penetrating injury to the tunica albuginea at sites other than the location of a penile hematoma may also be followed by shunt formation. Surgical correction of vascular shunts that occur following either type of trauma may be successful.
Figure 15.22 Cavernosogram of the penis of a bull demonstrating escape of contrast media into the peripenile circulation at the level of the distal sigmoid flexure.
Source: Courtesy of Dwight Wolfe.
Cases of erection failure due to multiple vascular shunts involving defects in the tunica albuginea of the free portion of the penis are thought to be the result of a congenital weakness in the structural integrity of the tunica. In such cases there is no history of penile trauma or injury. Affected bulls fail to achieve erection following sexual stimulation and may have bluish discoloration of the penile or preputial skin when attempting to breed or when attempts are made to induce erection with an electroejaculator. Discoloration is the result of blood exiting the CCP through the peripenile vasculature. Multiple distal vascular shunts are most often diagnosed in bulls during the first breeding season. Vascular shunts in the distal penis are readily demonstrable with cavernosography (see Figure 15.13). Unlike vascular shunts that form secondary to traumatic disruption of the tunica albuginea, surgical correction of multiple congenital shunts is unsuccessful.
Filling Defects of the CCP
Erection and maintenance of the normal penile form is dependent on complete distension of the unobstructed cavernous spaces with blood under pressure. If the cavernous spaces are blocked sufficiently to prevent blood from completely filling the CCP, engorgement of the penis can only progress from the proximal portion of the penis to the area of the filling defect [40]. The clinical presentation depends on the location of the obstruction. Complete obstruction of portions of the CCP may result in extension of the penis without erection of the portions distal to the blockage (Figure 15.23). Partial blockade of the cavernous spaces in the distal free portion of the penis may sometimes cause the erect penis to deviate ventrally or laterally.
Figure 15.23 Failure of the distal portion of the penis to become erect due to occlusion of the cavernous spaces of the corpus cavernosum penis.
Cavernosography can demonstrate filling defects in portions of the penis located distal to the sigmoid flexure. Filling defects may be congenital, secondary to fibrosis following trauma, or subsequent to cavernositis.
Denervation Injury
When mounting an estrus female the bull must position himself to make searching motions and use the penis to locate the vulva in order to make intromission. Sensory innervation to the glans and free portion of the penis is necessary for the bull to align the penis, achieve intromission, and successfully complete the breeding act. Without sufficient sensory innervation of the distal penis the bull is unable to locate the vagina and the coital act cannot be completed [41].
Sensory input from the distal penis is transmitted by branches of the paired dorsal nerves of the penis through the pudendal nerves to reach the spinal cord and brain. Although disruption of any portion of the neurologic pathway could result in loss of sensation, damage to the dorsal nerves of the penis is the most likely etiology of penile desensitization. Denervation injury does not interfere with erection, but affected bulls are unable to breed by natural service and usually cannot be successfully collected with an AV. Semen collection with an electroejaculator for artificial insemination is possible.
An observed test mating utilizing an estrus female in a confined area with adequate footing remains the best method to evaluate function of the dorsal nerves of the penis. Successful intromission and ejaculation definitively demonstrate normal nerve function [17]. In bulls with impairment of sensation to the distal penis, erection is not impaired but the searching motions made with the penis are ineffective and the bull will be unable to achieve intromission and complete the breeding act. Affected bulls often place the penis to the side of the tail head, on the cow's hip, or below the vulva near the udder (Figure 15.24). Observation of typical signs at the time of test mating coupled with a history of reproductive failure is usually sufficient for diagnosis, but definitive electrodiagnostic testing of nerve function [39, 42] is available at some veterinary teaching hospitals (Figure 15.25).
Figure 15.24 Failed attempt at copulation due to denervation injury of the penis. During repeated breeding attempts during test mating, the bull was unsuccessful in locating the vulva with the penis. Note the location of the penis beside the tail head of the cow.
Figure 15.25 Electrodes and stimulator in place for nerve conduction study of dorsal penile nerves. Electrical stimulation applied through the electrode on the free portion of the penis is recorded as the signal passes the proximally placed electrodes.
References
1 1 Rupp, G., Ball, L., Shoop, M., and Chenoweth, P. (1977). Reproductive efficiency of bulls in natural service: effects of male to female ratio and single‐ vs multiple‐sire breeding groups. J. Am. Vet. Med. Assoc. 171: 639–642.
2 2 Beckett, S., Hudson, R., Walker, D. et al. (1972). Corpus cavernosum penis
